DNA polymerases are extracted and purified from mammalian tissues. The low molecular weight DNA polymerase has been purifiid to homogeneity. The high molecular weight DNA polymerase are only partially purified, further purifications are being carried out. Since the low molecular weight enzyme is related to the high molecular weight enzyme immunologically, and preliminary evidences show some relationship between the low molecular weight and the mitochondrial enzymes, it appears likely that all these enzymes may contain some common subunits. Since the low molecular weight DNA polymerase consists of only one polypeptide chain, it appears possible that the low molecular weight DNA polymerase is the catalytic subunit of other DNA polymerases. Attempts are being made to establish the relationship of these enzymes by both biochemical and immunological techniques. Mechanisms of actions of isolated mammalian DNA polymerases and active site studies of the purified enzymes are being investigated.